The present invention is directed to a vortex flow meter and more particularly to the vortex detecting elements within the flow meter.
Vortex flow meter devices are generally operative to generate a frequency signal proportionate to the flow rate or velocity of a fluid passing through a conduit. In such devices a plurality of vortices are generated downstream from a vortex generator which is inserted into the fluid to be measured. There are a number of methods for detecting the vortices in terms of changes in pressure, flow velocity, transmission of ultrasonic waves, radiation of heat-sensitive elements or the like. However, an accurate frequency signal cannot be produced unless a sensor for producing such an electric output signal corresponding to the frequency of the aforementioned vortices is properly positioned within the conduit. An example of a method for properly positioning the sensor is explained in detail in Japanese Patent Publication No. 54-38513. An excellent method for measuring the frequency of Karman's vortices is a hot wire system which is highly responsive but inexpensive and is suitable over a wide flow rate range from a very low flow rate to a very high flow rate.
One of the problems with such a hot wire system is that the hot wires are easily broken when they are extended under tension since they are extremely thin. Also, if the hot wires are extended under tension directly on the conduit through which the fluid to be measured flows, the hot wires will break unless their thermal expansion is identical to that of the conduit for the fluid. Furthermore, the hot wires themselves tend to deteriorate rapidly when they are exposed directly to the pressure of the fluid passing through the conduit. In order to avoid the aforementioned problems, it has been proposed to deposit platinum or the like directly on the surface of the vortex generator. However, in such a method, the responsiveness of the hot wire system deteriorates since the heat of the deposited platinum is transferred to the vortex generator. In order to raise the temperature of the deposited conductive material an additional amount of current must be supplied which renders the system uneconomical.